Speciation is the generation of a new species by evolution. It is generally assumed that speciation is the result of isolation as observed by Darwin and Wallace on separate islands, allopatric speciation. More generally spoken reproductive isolation is required to stop the allele flow between the isolated groups. If this happens in what is essentially one population, such a reproductive isolation is called sympatric isolation or Wallace effect[1]. Many examples of sympatric isolation have been observed. They range from grasses, in which pollination time differences are observed [2], to birds, where song differences influence mating behavior [3][4]. As speciation can not so clearly separated between allopatric and sympatric, some intermediates are introduced parapatric and peripatric.

Controversies

Apart from problems to further discriminate the gradual differences between allopatric and sympatric. The more challenging questions are:

What is a population? Where it begins and where it ends?

What is a species.

The answers are by far not so trivial as it might be on the first glance.